In recent years, a photoreceptor system besides rods and cones has been discovered in the mammalian retina, consisting of a sub-population of retinal ganglion cells that express the visual pigment, melanopsin, and are intrinsically-photosensitive (ipRGCs). These ipRGCs comprise distinct subtypes. M1 cells have the strongest melanopsin-immunoreactivity, the highest intrinsic photosensitivity, and the largest saturated light response. M2 and M3 cells have weaker melanopsin-immunoreactivities, lower photosensitivities, and much smaller saturated light responses. M1, M2 and M3 cells also differ in the location of their dendritic arborizations in the inner plexiform layer of the retina. IpRGCs project predominantly to non- image-vision centers in the brain (e.g., most M1 cells), but they also project moderately to image-vision centers (e.g., many M2 cells). Finally, there are supposedly M4 and M5 ipRGCs, but these are not detectable with melanopsin-immunoreactivity, and have extremely low photosensitivities and small light responses. This proposal deals only with M1-M3 ipRGCs, which may have differential functions with respect to both non-image and image vision. In order to fully understand the melanopsin system, it is important to know in detail the ipRGCs' light responses and their underlying mechanisms. We have recently carried out extensive physiological/biophysical studies of M1 cells, and succeeded in resolving their single-photon response, estimating their melanopsin density, and molecularly identifying some of their key phototransduction components. This grant will be a continuation of these successful investigations. Aim 1 is to study M2 and M3 ipRGCs with respect to their intensity-response relations, response kinetics, single-photon-responses, membrane melanopsin densities, and their light-signaling thresholds, with the overall goal of comparing/contrasting M1, M2 and M3 cells. Aim 2 is to identify additional molecular components of phototransduction across M1, M2 and M3 cells, including the signaling G protein. We shall also examine the gating mechanism for the TRPC channels underlying the light response. Aim 3 is to begin to understand the termination mechanisms for the ipRGC's light response, including the significance of potential melanopsin phosphorylation, arrestin binding to melanopsin, and G- protein deactivation. Aim 4 is to study how M1, M2 and M3 ipRGCs adapt to steady light, and how this adaptation in the receptor current translates into action-potential firing and therefore signaling to the brain. We shall also investigate the involvement of Ca2+ in light adaptation. In summary, ipRGCs are the only known non-rod/non-cone photoreceptors in the mammalian retina. They are important for non-image vision and apparently also for subtle aspects of image vision. Therefore, learning in detail how they function is of fundamental importance to vision in both health and disease.